Electrodeposition of metals



Patented Jan. 7, 1936 ELECTRODEPOSITION OF METALS Louis Weisberg andWilliam B. Stoddard, In, New

York, N. Y., assignors to Weisberg & Greenwald, Inc., New York, N. Y., acorporation of New York No Drawing. Application March 30,1985, SerialNo. 13,922

22 Claims.

This invention relates-to new and useful improvements in methods forproducing electrodeposits of nickel, cobalt, or combinations of nickeland cobalt.

Ordinary electrodeposits of nickel, cobalt, and nickel-cobalt alloys aredull when they'are brought out of the plating solution, unless they arevery thin, and require bufling or polishing to produce a satisfactoryappearance. Since the bufllng or polishing operation is usually morecostly than the operation of electroplating itself, many attempts havebeen made to develop plating solutions which would produce deposits thatdo not require polishing or buffing. This has resulted in the appearanceof many so-called bright nickel plating solutions. These solutionsdiffer from one another, but all of them are subject to the objectionthat the deposits produced are brittle and prone to crack.

It may be well at this point to state that there are various ways oftesting anelectrodeposit for brittleness. In the case of some of theaddition agents that have been proposed, the only test necessary is toplate a piece long enough to put on a substantial thickness of plate(for example, 0.0005 inch or-upward); when the piece is removed from theplating solution, there are cracks in the deposit visible to the nakedeye. Sometimes the plate is subject to so much tension that the depositcracks and curls up, leaving patches entirely uncovered. Where thedeposit is apparently sound, one method of testing it for cracks is toexamine the surface by means of a microscope. Another method is to bringthe piece near the ear and bend it. If the plate is brittle, the soundof its tearing can be heard. Different deposits can be compared byobserving the angle to which the bend can be carried before a sound isheard. Still another method is to strip the deposit and test itdirectly. A good deposit, say one that is 0.001 inch thick, can be bentsharply on itself and handled quite roughly without damaging it; abrittle deposit on the other hand can be broken apart between thefingers with ease. Y

One object of the present invention is a method of producing lustrouselectrodeposits containing nickel, cobalt, or alloys of nickel andcobalt, and possessing little or no tendency towards brittleness andcracking. By lustrous we mean that the surfaces require no'bumng orpolishing after plating, and when the underlying surface is smooth,reflect images with mirror-like fidelity. These deposits are also freefrom cracks and other evidence of brittleness.

Another object of this invention is an efficient depolarizer for nickeland/or cobalt plating solutions whereby pitting and other defects in thedeposits can be eliminated.

Still further objects and advantages will appear from the more detaileddescription given below, it being understood, however, that this moredetailed description is given by way of explanation and illustrationonly, and. not by way of limitation, since various changes therein maybe made m by those skilled in the art without departing from the scopeand spirit of the present invention.

The nature and scope of our invention will appear from the followingaccount of the investigation which led up to it. Our original purpose 15was to develop a bright nickel plating solution and after manyexperiments and trials, we reached a point where we were able to obtainbright deposits, free from cracks and brittleness, from a solutioncontaining nickel sulphate, cobalt sul- 2o phate, and sodium formate,nickel sulphate being the predominant constituent. This solution gavereproducible results at room temperatures and low current densities(about 10-15 amperes per square foot), but when the temperature andcurrent density were raised to the usual range employed for rapidcommercial plating, it gave bright deposits for a while and then thedeposits became cloudy. After that no more bright deposits could beobtained from the solution. Nu- 3o merous attempts were made toresuscitate the solution, but all of them failed until it was discoveredthat a comparatively small addition of formaldehyde caused the solutionto give bright plate again and that the solution continued to givebright plate as long as small additions of formaldehyde were made fromtime to time.

Nevertheless, on attempting to convert an old nickel plating solution byadding the necessary ingredients to give the desired composition, brightplate was not obtained. ,A careful check-up re-' vealed that the onlydifference between the previous solution and this one was in the amountof ammonium salts present. Ammonium sulphate was present in bothsolutions, because both ammonia and sulphuric acid had been used inadjusting the-hydrogen ion concentration in the solutions. Furtherexperiments showed that there was an optimum amount of ammonium sulphatewhich should be present to give the best results.

' Finally, by adjusting the ammonium sulphate content to the properpoint in the old nickel plating solution that we were trying to convertinto a bright plating solution, we obtained excellent bright plate. a

. Having thus found a means of obtaining m trous electrodepositsconsisting of an alloy of nickel and cobalt in which nickel is the majorconstituent, we next set about investigating the eflectof varyingseparately each constituent in the plating solution and each variablesuch as temperature and current density. As'a conven- Iient means ofmaking such studies, the deposits were made on bent cathodes. Thetechnic of using the bent cathode is well known to those skilledin theplating art. The bent cathodes employed in this investigation werepiecesof nickel-silver about 2 inches long and slightly under of an inchwide, bent at right angles 1 inch from the bottom. The bent portionextended towards the anode when the cathode was in position for plating.The advantage of using a bent cathode is that it gives in one operationa picture of the effect of varying the current density over wide limits.

Because of the excellent throwing power and wide operating range ofthese solutions, most of the bent cathodes were plated at a currentdensity corresponding to an average of one hundred amperes per squarefoot. For solutions within the working limits which are hereinafterdescribed, the bent cathodes were bright all over at this currentdensity. As the limits for good plating were exceeded, dull or cloudyspots appeared, sometimes first in the recessed portions and sometimesat the tip and edges of the horizontal part of the cathode, according asthe current density was then too low or too high for bright plating. Insome cases, passing of the limits for good plating was evidenced bydarkening or burning of the deposit.

On some of the solutions the composition of which was such that they didnot give the best results, bent cathodes were also tried at a currentdensity corresponding to an average of sixty am- In addition, theeffects of adding boric acid and chloride, in the form of nickelchloride, were investigated to the extent that was necessary fordetermining whether or not their use was advisable. It was found thatneither boric acid nor chloride appears to have any appreciable efiecton the brightness of the deposit.

The cobalt content of the deposit increases with the cobalt content ofthe solution. As long as the. cobalt content of the solution is not toohigh, the relationship remains approximately linear. For a solutioncontaining approximately 240 grams of nickel sulphate and 45 grams ofnickel chloride per liter, we have found that the lower limit forcobalt, expressed in terms of cobalt sulphate, is in the neighborhood of10 to 12 grams per liter. As the amount of cobalt sulphate is furtherincreased, the deposit becomes-whiterall However, once the formatecontent is brought over the cathode, but particularly in the recessedportions, which with lower cobalt content in the solution have a darkercolor in comparison with the remainder of the surface.

A satisfactory working concentration in the above mentioned solution isobtained with 15' grams of cobalt sulphate per liter. The cobalt contentof a deposit made from a solution of this composition at an averagecurrent density of 60 amperes per square foot and at a temperature of 60degrees Centigrade is approximately fifteen per cent. As the cobaltcontent is raised beyond 15 grams per liter of cobalt sulphate,- thesolution continues to give bright deposits, and, we have observed noupper limit to the amount of cobalt 1 that may be employed, short ofplating practically pure cobalt. Of course, with the price of cobalthigher than that of nickel, as it is at present, there is a practicalreason for operating with the lowest possible proportion of cobalt. 2

Working with solutions of the composition described, we have found thatgood bright plate may be obtained on flat'surfaces with inclusions offormate radical as low as 20 grams of nickel formate per liter. such assodium formate, or cobalt formate may be used in place of nickelformate. Ammonium formate; if used, should be added only with dueconsideration for the specific effects exerted by ammonium salts. Theamounts of formate set 3 forth in the claims are expressed in terms ofnickel formate, Ni(CHO2)2.2H2O, but it is to be understood that thescope of our invention includes the use of other salts containingequivalent amounts of formate radical in place or instead of a nickelformate.

With the bent cathode, the lower range for good throwing power is foundto be around 30-40 grams of nickel formate per liter. We have found itconvenient to employ a working concentration 4 of 45 grams per liter,although we have operated satisfactorily solutions containing as highas. grams per liter. The formate content of the so lution appears toexert a considerable influence and to be critical at the lower end ofthe range. 4

above the critical range for good throwing power, a substantial furtherincrease has little effect.

The formate has a two-fold efiect. In the first place, it acts as anexcellent buffer for con- 5 trol of hydrogen ion concentration near theoptimum point. In addition to this, however, it has a specific action asa brightening agent when present in combination with certain of theother ingredients of the solution. This action is defi- 5 Otherequivalent soluble formates 2 nite and pronounced, for a solution madeup without formate and adjusted to the same hydrogen ion concentrationdoes not give as good results as a solution containing nickel orequivalent formate.

The optimum value of the hydrogen ion concentration, measuredcolorimetrically and ex pressed in terms of pH, is 4.2-4.3 for thesolutions above considered. The position of the optimum varies somewhatas the concentration of the difc5 ferent components is varied, butremains within a range of about 4 to 4.4. As the pH falls below 3.9, wehave found that the solution no longer produces bright plate unless thecurrent density is decreased. The solution then begins to smell 70 likea formic acid solution. Bright deposits have been obtained at a pH aslow as 2 with reduced current densities. On neutralizing the formicacid, good bright plate is once more obtained at the original currentdensity. It is desirable not 75 to use ammonia for neutralizing theacid, since? as has already been mentioned, a change inthe ammoniacontent of the solution produces a considerable effect on t e plate.When the pH is raised much above 4.5, he plate becomes dull and pitted.Onbringing the pH back within the working range by addition of sulphuricacid, the pitting disappears and the plate becomes bright again. 2Increasing the pH beyond the. upper working range causes a precipitateto separate out whenthe solution is cooled. This precipitate, webelieve, consists of nickel and/or cobalt formate.

We have found that the addition of ammomum salts not only affects thebrightness of the plate. but also has a marked effect on the physicalproperties of the plate itself. Successive increases in the amount ofammonium salts present with other components constant produce first abrightening of the deposit,and then after the optimum has been passed, avery decided darkening, until finally the deposit becomes gray-black.With lower ammonium salt content, the plate is soft and ductile; as theammonium salt content is increased, the deposits become harder, and whenthe point is reached where the deposit becomes dark or black as has justbeen described, the deposit has also become brittle. It is important,therefore, to avoid getting too much ammonia or ammonium salts in thesolution.

, Starting with a solution containing no ammonium salts, we find thataddition of even one or two tenths of a gram per liter produces animmediate brightening effect. A solution containing even this amount ofammonium salts is more stable and permanent in operation than asolutlon-inwhich ammonium salts are not present. Ammonium sulphate,ammonium chloride, and ammonium borate are convenient salts to use. jApparently the effect of different salts is in proportion to theirammonia content, or at least approximately so. The quantities of ammo--nium salts are expressed here and in the claims in terms of ammoniumsulfate, but it is to be understood that equivalent amounts of otherammonium salts may be used in, place or instead of ammonium sulfate. Agood working concentration is obtained when the ammonium sulphatecontent exceeds 0.5 or preferably 0.7 grams per liter, but does notexceed 5 grams per liter. The higher concentrations of ammonium sulphateproduce much harder deposits. If the, deposits are to be subjected toany kind of mechanical working, it is desirable to work near the lowerend of the range, say at about 1 gram per liter of ammonium sulphate.However, for most purposes, a working concentration of two to threegrams per liter of ammonium sulfate'is prefer-- able, because with thiscomposition the solution has a broader working range. When the ammoniumsulphate exceeds 5 grams per liter, the deposits begin to lose inbrightness and show a tendency towards brittleness. Above 15 grams perliter, the deposits become so brittle that they 5 crack and crumble topieces. When the ammo- I nium salt content of the solution approachesthe upper working limit, its dulling effect may be counteracted in partby lowering the pH. We

have also found that increasing the formaldehyde 0 content has somebeneficial effect in this situa- Men.

#With ammonium borate, for example, instead of ammonium sulphate, arecommended concentration would be in the neighborhood of 4 grams 's-per liter. A solution containing 3.6 grams per liter has been found togive excellent resirts with respect torig litn of the deposit and topossess good throw ng fir at an average current density of 100 amperesper-square foot.

Whe maldehyde is added in successive small amo a plating solution suchas has been descri ed eeping all other factors constant, thereMisanimmediate brightening effect at first; finally a point is reachedwhere further additions cause the brightness to diminish. At the upperlimit, the odor of formaldehyde becomes so pronounced that it would notbe convenient to use. such high concentrations. The lower limit for therecommended workingconcentration is about 0.5 gram per liter of actualformaldehyde. The upper limit is about 7.5 grams per liter. Sincecommercial solutions of formaldehyde contain approximately 40% by weightof formaldehyde, the recommended minimum is equivalent to about 1.25cubic centimeters of commercial formaldehyde solution per liter.

We have found that paraformaldehyde can be used in place offormaldehyde. The beneficial effect of formaldehyde and certain of itsimmediate derivates such as paraformaldehyde is a specific one.Certain'other aldehydes such as benzaldehyde and furfuraldehyde, on thecontrary, produce black deposits that are utterly useless. We have foundthat whilehexamethyl ene tetramine causes brightening of the deposit, 80it also causes severe embrittlement.

In general, we prefer to use a solution having the followingcomposition, although variations within the limits hereinbeforementioned afford satisfactory results: as

' Grams per liter Nickel sulphate 240 Nickel chloride Nickel formate l:45 .Cobalt sulphate 15 4o Ammonium sulphate 1 Formaldehyde 0.5-1.0

A solution having the following composition produces a deposit somewhatless ductile, but 5 this solution has a broader plating range:

Grams per liter Nickel sulphate 240 Nickel chloride 45 Sodium formate -135 Cobalt sulphate 15 Ammonium sulphate 2.5 Formaldehyde 1.0

The total metal content (nickel plus cobalt), expressed as thesulphates, may be varied from 120 to 400 grams per liter, so long as theratio between the nickel and the cobalt is kept within suitable limits.These limits are wide. A ratio of 20 nickel to l cobalt gives excellentresults; so

does 10 nickel to 1 cobalt. On the other hand, fair results have beenobtained with a ratio of 20 cobalt to 1 nickel. The chloride expressedas metal chloride may vary all the way from 0 to grams or more perliter. The purpose of the chloride is to produce adequateanodecorrosion. For high purity nickel anodes 45 grams per liter or more arerecommended. Cobalt anodes or anodes containing cobalt may be operatedwith a lower chloride content. The formate radical may be varied fromabout 20 to grams, in terms of nickel formate, per liter; at the upperlimit the viscosity of the solution begins to become noticeablyafi'ected, although there is apparently no deleterious effect otherwise.The'ammonium sulphate may vary from about 0.5 to 5 grams or 7 usestraight nickel anodes and add cobalt sul-' phate as required from timeto time. Only oc-' casional analyses are needed for checking the cobaltcontent. Additions of cobalt, sulphate can be made with sumcientaccuracy over long periods of time by calculation from the number ofampere hours passed through the plating solution. The current efliciencyis almost one hundred per cent.

In plating with the preferred solutions above described, we prefer tomaintain the pH value between 4.2 and 4.3, the temperature between 50and 65 degrees centigrade, and the current density between 50 and 150amperes per square foot The solution must be filtered regularly to keepit free from suspended matter. The solution should be agitated or thework to be plated should be moved as is usually done in plating withwarm nicked solutions.

The deposits obtained under these conditions have a brillant lustre. Asthe thickness of the deposit is increased, the lustre increases. Evenwhen the underlying metal has a surface which is not smooth or wellpolished to begin with, the,

surface becomes more and more lustrous as the plating proceeds and thefinal appearance is greatly improved if sufiicient thickness of depositis applied. This contrasts with the usual result, in which surface marksand defects are accentuated.

Boric acid in varying amounts is present in most nickel platingsolutions. Such solutions can usually be converted into a, solutionhaving the above stated composition by proper adjustment and additions.The boric acid present in such cases is not harmful .and can be dsregarded.

When cobalt sulphate is omitted, the nickel plating solution whichremains gives nickel deposits which are somewhat cloudy. The results canbe improved by adjusting the concentration of the various components, asfor example, by increasing the nickel formate to 100 grams per liter anddecreasing the ammonium sulphate to 0.5. gram per liter, but the beststraight nickel deposits that we have obtained do not compare inbrilliance with the results that are obtained on the addition of cobalt.It is noteworthy, nevertheless, that such nickel deposits are unusuallysmooth and free from elevations or depressions of any kind. They show atmost a haze or cloud, and are remarkably free from any tendency towardspitting. We have found that nickel plating solutions containingformaldehyde and ammonium salts in proper amounts, with or withoutformate, are to be preferred to the old style nickel plating solutioneven when the plate is not required to be bright. Simlliar results areobtained from solutions containing cobalt without nickel. on. addingnickel to such solutions, greater brightness is obtained.

The action of formaldehyde is evidently a depolarizing action, for ithas been observed that in its presence hydrogen evolution is apparentlycompletely stopped. This is in accord with the fact that pitting isknown to be connected with hydrogen evolution and that preventinghydrogen evolution reduces pitting it also follows that the regularityof thereflecwhich is just on the border line of being fully 2i While wedo not wish to be bound by any theory as to what takes place in thesesolutions, we believe that the key to an explanation of the productionof bright plate is to be found in this depolarizing action, and theconsequent absence of elevations or depressions in the plated surface.It is well known that if a surface is perfectly plane, mirror-likereflection is obtained. According to the wave theory of light, a surfacereflects like a plane surface as long as there are 1 no elevations ordepressions in it which have dimensions exceeding a small fraction ofthe wave length of light. For a given degree of roughness,

tion depends not only on the wave length of the 1 light but also ontheangle of incidence. The smoothness of the surface is therefore besttested with light falling on it at nearly grazing, incidence. This maybe seen by examining a plate bright. When looked at in the usual way, itseems bright, but if it is held so that it can be viewed by lightfalling on it at nearly grazing incidence, it shows a faint haziness orcloud. Perfectly bright plate, though, does not show this 2; cloud.

All the observed facts are consistent with the hypothesis that themechanism by which bright plate is produced is simply that which isinvolved in the elimination of pitting and treeing. car- 30 ried outmore perfectly,'of course, than is the case in ordinary plating.

We claim:

1. The method of electrodepositing nickel which comprises electrolyzinga solution contain- 85 ing a nickel salt from the group consisting ofsulphates and chlorides, in the presence of an ammonium salt, a formateand formaldehyde.

2. The method of electrodepositing nickel which comprises electrolyzinga'solution contain- 40 ing a nickel salt from the group consisting ofsulphates and chlorides,in the presence of an ammonium salt, a formate,and formaldehyde, and having a pH of about 4 to 4.4.

3. The method of electrodepositing a nickelcobalt alloy which compriseselectrolyzing a solution containing nickel and cobalt salts from the Ygroup consisting of sulphates and chlorides, in the presence of anammonium salt, a formate and formaldehyde. 60 l 4. The method ofelectrodepositing a nickel cobalt alloy which comprises electrolyzing asolution containing nickel and cobalt salts from the group consisting ofsulphates and chlorides, in the presence of an ammonium salt, a formate,and formaldehyde, and having a pH of about 4 to 4.4.

5. The method of electrodepositing nickel which comprises electrolyzinga solution containing a nickel salt in the presence of 0.5 to 5 gramsper liter of an ammonium salt, 20 to 100 grams per liter of a formate,and 0.5 to 7.5 grams per liter of formaldehyde.

6. The method of electrodepositing nickel which comprises electrolyzinga solution containing a nickel salt in the presence of 0.5 to 5 gramsper liter of an ammonium salt, 20 to 100 grams per liter of a formate,and 0.5 to 7.5 grams per liter of formaldehyde, the pH of the solutionbeing about 4 to 4.4.

7. The method of electrodepositing a nickelcobalt alloy which compriseselectrolyzing a solution containing nickel and cobalt salts in thepresence of 0.5 to 5 grams per liter of an animonium salt, 20 to gramsper liter of a formate,

and 0.5 to 7.5 grams per liter of formaldehyde. 16

8. The method of electrodepositing a nickelcobalt alloy which compriseselectrolyzing a solution containing nickel and cobalt salts in thepresence of 0.5 to 5 grams per literof an ammonium salt, 20 to 100 gramsper liter of a formate, and 0.5 to 7.5 grams per liter of formaldehyde,the pH of the solution being about 4 to 4.4.

9. The method of electrodepositing nickel which comprises electrolyzinga solution containing a nickel salt from the group consisting ofsulphates and chlorides, in the presence of an am-' monium salt andformaldehyde.

10. The method of electrodepositing a nickelcobalt alloy which compriseselectrolyzing a solution containing nickel and cobalt salts from thegroup consisting of sulphates and chlorides, in the presence of anammonium maldehyde.

11. The method of electrodepositing nickel which comprises electrolyzingan acid solution containing a nickel salt from the group consisting ofsulphates and chlorides, in the presence of formaldehyde.

12. The method ofelectrodepositing a nickelcobalt alloy which compriseselectrolyzing an acid solution containing nickel and cobalt salts fromthe group consisting of sulphates. and chlorides. iii the presence offormaldehyde.

18. An electrolyte for depositing bright metal coatings consisting of awater solution containing salts of the metals nickel and cobalt in theproportion expressed in terms of sulphate of 120 to 400 grams perliter,-a formate in the proportionof 20 to 100 grams per liter, anammonium salt in the proportion of 0.5 to 5 grams per liter, andformaldehyde in the proportion of 0.5 to 7.5

grams per liter.

14. An electrolyte for depositing bright metal coatings consisting of awater solution containins salts of the metals nickel'and cobalt in theproportion expressed in terms of sulphate of'120 to 400 grams per liter,0. format'e in the proportion of 20 to 100 grams per liter, an ammoniumsalt in the proportion'of 0.5 to 5 grams per liter,

and formaldehyde in the proportion of 0.5v to 1.5 grams per liter, thepH being adjusted to about 15. An electrolyte for coatings consisting ofa water solution containing 240 grams per liter of nickel sulphate. 45grams perliter of nickel chloride. grams per liter of salt and forcobaltsulphate, 45 grams per liter of nickel formate, 1 gram per liter ofammonium sulphate, and 0.5-1.0 gram per liter of formaldehyde.

'16. An electrolyte for depositing bright metal coatings consisting'oi'a water solution containing 240 grams per liter of nickel sulphate, 45

grams per liter of nickel chloride, 15 grams per liter of cobaltsulphate, 45 grams per liter of nickel formate, 1 gram per liter ofammonium sulphate, and 0.5-1.0, gram per liter of formaldehyde, thesolution having a pH of 4.2 to 4.3.

' 17. The method of electrodepositing a metal from the class consistingof nickel and cobalt, which comprises electrolyzing an acid solutioncontaining a salt from the class consisting of the sulphates andchlorides of the metals of said group, in the presence of a formate, anammo- 30 nium salt and formaldehyde.

20. An electrolyte for depositing a coating of a metal from the classconsisting of nickel and cobalt, which consists of a water solutioncontaining a salt from the class consisting of the sulphates andchlorides of the metals of said group, and formaldehyde.

21. An electrolyte for depositing a coating of a metal from the classconsisting of nickel and cobalt, which consists of a water solutioncontaining a salt from the class consisting of the sulphates andchlorides of the metals of said group, an ammonium salt andformaldehyde.

22. An electrolyte for depositing a coating of a metal from the classconsisting of nickel and cobalt, which consists of a water solution con-'tainingasaltfromtheclassconsistingofthesuldepositing bright metalphates and chlorides of the metals of said group, a formats, an ammoniumsalt and formaldehyde.

LOUIS WEIBBERG -wn.1.mun.s'ro' .a.

